The present invention relates to a unidirectional DC-DC converter for converting an inputted DC voltage into a DC voltage of different magnitude, and a controlling method therefor.
In a buck-boost DC-DC converter for outputting an inputted DC voltage such that this inputted DC voltage is converted into a DC voltage of desired magnitude, implementation of high efficiency in this converter is made possible by lowering the switching loss with the use of soft switching technology. In accompaniment with this implementation, passive elements, such as inductor and capacitor, can be downsized by achieving implementation of high frequency in the driving frequency for switching elements.
In Grover Victor Torrico Bascope, “Single-Phase High Power Factor Variable Output Voltage Rectifier, Using the Buck+Boost Converter: Control Aspects, Design and Experimentation”, the following unidirectional DC-DC converter is disclosed: A buck converter and a boost converter, which co-use a main inductor, are connected in series with each other. Then, the buck converter and the boost converter are selectively driven, depending on a comparison result as to which is larger of an input voltage and an output voltage. The basic configuration of the main circuit includes, first, a DC power-supply and a buck converter circuit, the buck converter circuit including a first main switching element for intermittently intercepting and connecting a current flowing into a DC load via the main inductor from the DC power-supply. The basic configuration includes, next, a boost converter circuit, the boost converter circuit including a second main switching element for short-circuiting the DC load, and intermittently intercepting and connecting a current in a circuit for accumulating energy into the main inductor from the DC power-supply. Here, first and second snubber capacitors are connected in parallel to the first and second main switching elements each. Also, diodes are connected in inversely parallel to the first and second main switching elements each. Moreover, the basic configuration further includes a control device for turning ON/OFF the first and second main switching elements, and controlling duties of the main switching elements, and an output diode for releasing the energy accumulated into the main inductor onto the load side by the ON/OFF operation of the main switching elements.
Next, in Tsuruta et al., “Proposal of 98.5% High Efficiency Chopper Circuit QRAS for the Electric Vehicle and the Verification”, IEEJ Trans. IA, Vol. 125, No. 11, 2005, the following zero-current switching (ZCS) scheme is disclosed: In a unidirectional DC-DC converter of boost-chopper type, current change ratios of main switching elements are suppressed by an inductor.
Also, in JP-A-2005-318766, a soft-switching-capable unidirectional DC-DC converter is disclosed. Here, after main switching elements are directly connected to a DC power-supply, an auxiliary resonant circuit including an auxiliary switching element is connected to a DC circuit of the DC power-supply and the main switching elements in a buck converter circuit where an inductor and a load are connected in series.
Also, in JP-A-2006-14454, the following technology is disclosed: As is the case with JP-A-2005-318766, in a buck converter circuit, an auxiliary resonant circuit including an auxiliary switching element and a transformer is connected to a DC circuit of a DC power-supply and main switching elements.
Meanwhile, JP-A-6-311738, the following technology is disclosed: In a unidirectional DC-DC converter having a boost converter circuit, a DC circuit of an auxiliary switching element, auxiliary inductors, and a diode is connected to a DC circuit of a DC power-supply and main to switching elements. Here, the auxiliary inductors are magnetically coupled to a main inductor. Also, the auxiliary switching element is turned ON before the main switching elements are turned ON.
By the way, if no modification is made to the main circuit of the unidirectional DC-DC converter which is disclosed in Grover Victor Torrico Bascope, “Single-Phase High Power Factor Variable Output Voltage Rectifier, Using the Buck+Boost Converter: Control Aspects, Design and Experimentation”, switching losses in the main switching elements are tremendous in amount. As a result, there exists a drawback that the implementation of high frequency is difficult to achieve, and that the device dimension becomes large.
Meanwhile, in the main circuit configurations of the unidirectional DC-DC converters, each of which is disclosed in Tsuruta et al., “Proposal of 98.5% High Efficiency Chopper Circuit QRAS for the Electric Vehicle and the Verification”, IEEJ Trans. IA, Vol. 125, No. 11, 2005, JP-A-2005-318766, JP-A-2006-14454, and JP-A-6-311738, control ranges of the output voltages are biased into some range or other with respect to the DC power-supply voltage. This shortcoming limits application targets to which the main circuit configurations are applicable. Namely, in the boost converter circuits disclosed in JP-A-2005-318766 and JP-A-2006-14454, there exists a drawback that no control cannot be exerted over the output voltages which are lower than the DC power-supply voltage. Also, in the buck converter circuits disclosed in Tsuruta et al., “Proposal of 98.5% High Efficiency Chopper Circuit QRAS for the Electric Vehicle and the Verification”, IEEJ Trans. IA, Vol. 125, No. 11, 2005, and JP-A-6-311738, there exists a drawback that no control cannot be exerted over the output voltages which are higher than the DC power-supply voltage.
Also, in JP-A-2005-318766 and JP-A-2006-14454, from their basic principle, the large inductors, each of which is equivalent to more than one-half of the main inductor, are required as the resonating (auxiliary) inductors. As a result, there exists a drawback that dimension/weight of the auxiliary inductors become increased.
Moreover, in JP-A-2006-14454, there also exists a drawback that, from the use of the transformer and its relationship with the circuit configuration, occurrence of voltage surge is feared due to influences of leakage inductance.